01 - Preheat the oven to 350°F. Grease a 9x13-inch baking dish with butter or nonstick cooking spray.
02 - Arrange the torn croissant pieces evenly across the bottom of the prepared baking dish. Scatter the mixed berries over and between the croissant pieces, distributing them uniformly.
03 - In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, granulated sugar, vanilla extract, lemon zest if using, and salt until smooth and well combined.
04 - Pour the custard mixture evenly over the croissants and berries. Gently press down on the croissant pieces to ensure they absorb the custard thoroughly.
05 - Sprinkle the turbinado or granulated sugar evenly over the surface to create a light, crispy topping during baking.
06 - Allow the assembled bake to rest for 10 minutes so the croissants fully soak up the custard. If the croissants are very fresh, extend the resting time for better absorption.
07 - Bake uncovered for 35 to 40 minutes until the custard is fully set and the top is golden brown.
08 - Let the bake cool for 10 minutes. Dust with powdered sugar if desired and serve warm.